1. Field of the Invention
The invention is related to the field of flow meters, and in particular, to proving flow meters.
2. Description of the Prior Art
Coriolis flow meters are “synthesized pulse devices” as defined by the American Petroleum Industry (API). This simply means that it takes a finite amount of time to calculate the flow that has passed through the meter. In a typical Coriolis flow meter, it takes a short but measurable time to calculate the flow that has passed through the meter. This causes a time delay between the actual flow and the measured flow. The results of the time delay is a flow reading that is offset or delayed from the actual flow by the time delay. In most uses this delay does not cause problems because at the end of the batch, after waiting a short time, the correct value for flow through the meter is available.
Proving is a field calibration method where a known volume is flowed through the flow meter and compared to the flow measured by the flow meter. Provers can be stationary, e.g. mounted permanently next to the meter, or truck mounted so that the prover can calibrate a plurality of meters. A typical prover is a device with a pipe (104) of a known internal diameter. A ball or piston (102) slides inside the pipe (104) and passes two sensors (S1,S2) or detectors. The first sensor (S1) signals the prover's computer to start counting pulses from the flow meter being calibrated. The pulses are typically proportional to volume flow rate. The second sensor (S2) signals the prover to stop counting pulses from the meter being calibrated. The volume inside the pipe between the two detectors is well known and often compensated for pressure and temperature. The total volume between the two detectors is compared to the number of pulses from the flow meter and a meter factor is determined. The meter factor is simply a correction factor applied to the meter's output. Depending on the volume of the prover and the flow rate used, the measurement time for the known volume to pass through the flow meter may be from 0.5 seconds to 60 seconds. This procedure works well for many types of flow meters (turbine meter, PD meter, etc.) but may have problems with synthesized pulse devices due to the time delay in the measured flow. Because the measured flow is offset in time from the actual flow, the prover is comparing flow volumes from different instances in time.
Any change in flow rate during the calibration test can cause a difference in the measured flow between the different instances in time. One cause of flow rate change is when the prover launches the ball or piston. A change in pressure may occur when the ball or piston is launched, causing a change in flow rate. The prover measures the new flow rate, and the Coriolis flow meter measures a weighted average of the old flow rate and the new flow rate, causing an error between the two measurements. Provers often have a length of pipe that the ball or piston travels before crossing the first detector. This length of pipe is typically called the “prerun”. The prerun length is equivalent to a fixed volume. The prerun time is dependent on flow rate. At high flow rates, the prerun time may not be long enough to completely stabilize the flow rate. Even at low flow rates, the flow rate through the prover may not be completely stable. This is not a problem for many types of flow meters but it may cause an error when proving a synthesized pulse devices.
Therefore there is a need for a system and method for proving synthesized pulse devices.